This invention claims priority to Japanese patent application JP 2002-128629, the disclosure of which is incorporated herein by reference.
This invention relates to a window member used in a viewing window of a combustion apparatus using wood, gas, coal, kerosene, or the like as a fuel.
As a combustion apparatus of the type, a heater, a cooker, a boiler, and the like are known. For example, a fireplace-type stove as the heater is generally classified into a direct heating system and an indirect heating system. The stove of the direct heating system is opened at its front portion and directly heats an area around the stove by heat radiation from a combustion flame.
On the other hand, the stove of the indirect heating system heats air by an internal heat source into warm air and supplies the warm air into a room by the use of a fan or blower so that a whole of the room is heated by circulation of the warm air. The stove of the indirect heating system has a viewing window on its front side. Through the viewing window, the combustion flame can be monitored and confirmed. Therefore, it is possible to keep safety against occurrence of fire and production of a toxic gas resulting from incomplete combustion. In addition, sensuous heating effect or warm feeling is given because the combustion flame is seen through the viewing window.
A window member used in the viewing window is required to have heat resistance against 300° C. or more. Generally, the window member of the type is made of a low expansion glass such as a fused silica, or a low-expansion borosilicate glass, or a crystallized glass with β-quartz solid solution precipitated as main crystals.
However, if the combustion apparatus using wood, gas, coal, kerosene or the like as a fuel is used for a long time, ash dust is adhered to the window member so that the window member becomes remarkably dirty. Thus, the appearance of the combustion apparatus is spoiled.